1. Field of the Invention
The present invention relates to a liquid crystal display device using a ferroelectric liquid crystal and a method of driving a ferroelectric liquid crystal display element.
2. Description of the Related Art
A ferroelectric liquid crystal display element has advantages of a high-speed operation and a wide field angle over a TN-mode liquid crystal display element using a nematic liquid crystal.
Studies on practical applications of ferroelectric liquid crystal elements have been conventionally concentrated on a ferroelectric liquid crystal called an SS-F liquid crystal. The SS-F liquid crystal is a ferroelectric liquid crystal having a chiral smectic C-phase whose helical pitch is larger than the substrate distance (cell gap) of a liquid crystal element, and having a memory function (bistable state) in an aligned state.
The SS-F liquid crystal is sealed between the substrates of a liquid crystal display element in a state wherein the helical structure is not manifested. When a voltage of one polarity is applied to the SS-F liquid crystal, it is set in the first aligned state in accordance with the interaction between the applied voltage and spontaneous polarization. When a voltage of the other polarity is applied, the SS-F liquid crystal is set in the second aligned state. In the ferroelectric liquid crystal display element using the SS-F liquid crystal, the transmittance of light is controlled in accordance with the first and second aligned states and a pair of polarizing plates disposed on the incident and exit sides of the element, thereby displaying an image.
The aligned states of the SS-F liquid crystal are only the first and second aligned states. For this reason, the transmittance cannot be changed stepwise in the ferroelectric liquid crystal display element using the SS-F liquid crystal, and gradation display cannot be performed.
Ferroelectric liquid crystal display elements capable of performing gradation display have been studied, and use of a ferroelectric liquid crystal having a chiral smectic phase whose helical pitch is smaller than the substrate distance of a display element is proposed. A ferroelectric liquid crystal of this type having a memory function is called an SBF liquid crystal, and that having no memory function is called a DHF liquid crystal ("LIQUID CRYSTALS", 1989, Vol. 5, No. 4, pp. 1171-1177).
In a ferroelectric liquid crystal display element using an SBF liquid crystal, the SBF liquid crystal in a state of a helical structure is sealed between two substrates. When a voltage having an absolute value equal to or larger than a predetermined value is applied across electrodes through a liquid crystal layer, the SBF liquid crystal is set in the first or second aligned state in accordance with the polarity of the applied voltage. The first aligned state is a state in which the direction (director) of the long axis of each liquid crystal molecule is aligned almost parallel to the first direction, and the second aligned state is a state in which the director of each liquid crystal molecule is aligned in a direction almost parallel to the second direction. When the absolute value of the applied voltage is less than the predetermined value, the SBF liquid crystal is set in an intermediate aligned state wherein liquid crystal molecules whose directors are aligned in the first direction and liquid crystal molecules whose directors are aligned in the second direction are mixed.
In the ferroelectric liquid crystal display element using a DHF liquid crystal, the DHF liquid crystal in a state of a helical structure is sealed between substrates. When a voltage having an absolute value equal to or larger than a predetermined value is applied across electrodes through a liquid crystal layer, the DH liquid crystal is set in the first or second aligned state in accordance with the polarity of the applied voltage. The first aligned state is a state in which the director of each liquid crystal molecule is aligned in a direction almost parallel to the first direction, and the second aligned state is a state in which the director of each liquid crystal molecule is aligned in a direction almost parallel to the second direction. When the absolute value of the applied voltage is less than the predetermined value, the DHF liquid crystal is set in an intermediate aligned state wherein the average direction of the directors of the liquid crystal molecules is an intermediate direction between the first and second directions due to distortion in the helical structure of the molecular arrangement.
For this reason, if a voltage for maintaining the intermediate aligned state is kept applied to the liquid crystal display element of an active matrix type even during a nonselection period, gradation display using the SBF or DHF liquid crystal may be performed.
In practice, however, even if a voltage corresponding to a gradation level to be displayed is applied to the liquid crystal display element of the active matrix type, the applied voltage does not correspond to the transmittance of the pixels. As a result, gradation display in a practical sense cannot be realized due to the following reason. The optical characteristics (i.e., the relationship between the applied voltage and the transmittance) of the SBF and DHF liquid crystals have a large hysteresis. Even if a voltage corresponding to a display gradation level is applied to the liquid crystal, the display gradation level cannot be uniquely determined due to an influence of the past voltage applied to the element.
When the ferroelectric liquid crystal display element is of an active matrix type, the voltage held in each pixel cannot become zero immediately upon a power-OFF operation. For this reason, even after the power switch is turned off, the aligned state corresponding to the held voltage of the pixel is held in the liquid crystal to cause a display burn-in phenomenon.